Γ-valley assisted intervalley scattering in monolayer and bilayer WS2 revealed by time-resolved Kerr rotation spectroscopy

Abstract

We investigated the processes of valley depolarization in monolayer and bilayer $\mathrm{W}{\mathrm{S}}_{2}$ with polarization-resolved photoluminescence (PRPL), time-resolved Kerr rotation (TRKR), and polarization-resolved differential reflectance (PRDR) spectroscopies. We found that with increasing energy of excitation from 2.14 to 2.38 eV, the degree of circular polarization of steady-state PL decreases in monolayer $\mathrm{W}{\mathrm{S}}_{2}$. However, the TRKR and PRDR spectra show some contrasting behaviors: when the pump energy is beyond 2.25 eV, a slow decay of TRKR signals gradually appears and increases with the increased excitation energy. This relaxation time is estimated to be around 30 ps at the excitation energy of 2.34 eV. We explain this paradox with a simple model of a three-level system. This slow relaxation process in the Kerr rotation spectra is attributed to the $\mathrm{\ensuremath{\Gamma}}$-valley assisted intervalley scattering between $K$ and $K$\ensuremath{'} valleys under the off-resonant excitation. For bilayer $\mathrm{W}{\mathrm{S}}_{2}$, an even slower decay ($48\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$) of TRKR signals is observed, which is comparable to the exciton lifetime ($58\ifmmode\pm\else\textpm\fi{}0.6\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$). The prolonged valley lifetime is attributed to the irreversible scattering process from the $K$ (or $K$\ensuremath{'}) to the $\mathrm{\ensuremath{\Gamma}}$ valley.